1. Field of the Invention
This invention relates to the field of Fischer-Tropsch products and more specifically to the field of blending Fischer-Tropsch products with hydrocarbons.
2. Background of the Invention
Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical for formations containing small amounts of natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas, mixtures of C1-C2 hydrocarbons or methane, the major chemical component of natural gas, is reacted with oxygen, or steam, or carbon dioxide, or any mixture of two or more thereof, to form synthesis gas (also called syngas), which is a combination of carbon monoxide gas and hydrogen gas. The first transformation may comprise steam reforming, auto-thermal reforming, dry reforming, advanced gas heated reforming, partial oxidation, catalytic partial oxidation, combinations thereof, or other processes known in the art. The first transformation to make syngas may be facilitated by a catalyst. Catalyst compositions useful for synthesis gas reactions are well known in the art. They generally are comprised of a catalytic metal selected from Groups 8, 9, and 10 of the Periodic Table (new IUPAC notation) such as noble metals. The catalytic metal may be supported on monoliths, wire mesh and/or particulates of refractory carriers.
The second transformation, known as the Fischer-Tropsch synthesis, generally entails contacting the synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Any Fischer-Tropsch technology and/or methods known in the art will suffice; however, a slurry bubble reactor is preferred. The feed gas charged to the second transformation comprises synthesis gas and optionally an off-gas recycle from the same or another Fischer-Tropsch process. It is preferred that the molar ratio of hydrogen to carbon monoxide in the feed gas be greater than 0.5:1 (e.g., from about 0.67 to about 2.5). The temperature of the second transformation is typically in the range from about 160° C. to about 350° C.
Fischer-Tropsch catalysts are well known in the art and generally comprise a catalytically active metal, a promoter and optionally a support structure. The most common catalytic metals are Group 8, 9 and 10 metals of the Periodic Table (new IUPAC Notation), such as cobalt, nickel, ruthenium, and iron or mixtures thereof. The preferred metals used in Fischer-Tropsch catalysts with respect to the present invention are cobalt, iron and/or ruthenium; however, this invention is not limited to these metals or the Fischer-Tropsch reaction. Other suitable catalytic metals include Groups 8, 9 and 10 metals. The promoters and support material are not critical to the present invention and may be comprised, if at all, by any composition known and used in the art. Promoters suitable for Fischer-Tropsch synthesis may comprise at least one metal from Group 1, 7, 8, 9, 10, 11, and 13. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. The products of the Fischer-Tropsch synthesis may include a large range of molecular weights from light hydrocarbons such as methane to very large molecules with 50 or more carbon atoms. Therefore, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. The Fischer-Tropsch product primarily comprises normal paraffins. It generally has very low contents of heteroatomic impurities such as sulfur-containing compounds, nitrogen-containing compounds or metals. The hydrocarbon product contains practically no aromatics, naphthenes or, more generally, cyclic compounds, in particular when cobalt catalysts are used. While hydrocarbon streams produced via Fischer-Tropsch synthesis may be used in a variety of applications, their use as liquid fuels is of significant interest. In particular, Fischer-Tropsch products are suitable for production of high cetane and low emissions diesel fuels. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield naphtha, as well as middle distillates. Hydrocarbon waxes may be subjected to an additional processing step (typically a hydrocracking step) for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to obtain primarily hydrocarbons that are liquids and waxes, which are nongaseous hydrocarbons (e.g., C5+ hydrocarbons).
Fischer-Tropsch products have also been used to blend with hydrocarbon products. In the hydrocarbon industry, hydrocarbon products may be used as a plurality of fuels. For instance, hydrocarbons are typically used as diesel fuels. However, to be used as a diesel fuel, the hydrocarbon products typically have specification standards to meet such as industry standards, environmental concerns, government regulations, and the like, which require the hydrocarbon product to have density properties within a certain range. Specification standards may also require that other properties such as sulfur content, aromatics content, boiling point range, and the like be within required ranges. The hydrocarbon products can include refinery product streams such as light cycle oils, vacuum gas oils, heating oils, and the like. These product streams typically have densities that are not within the specification standards for diesel fuels. Therefore, it is highly advantageous to lower the density of these hydrocarbon product streams and thereby increase the potential uses of such refinery product streams for higher-value markets.
Lower density fuels such as kerosene, jet fuel and the like have been used in the past to reduce the density of the hydrocarbon product streams. Jet fuel and kerosene are typically blended with the hydrocarbon product in amounts to bring the hydrocarbon product within a desired density range. The jet fuel and kerosene can be independently blended with the hydrocarbon product stream or can both be blended with the hydrocarbon stream. Drawbacks to blending with the lower density streams include the hydrocarbon product stream having properties that may not be able to satisfy other specification standards. For instance, blending a hydrocarbon product stream with a kerosene may bring the hydrocarbon product stream within density specification standards but not within sulfur or flash point specification standards. Further drawbacks include the cost efficiency of the lower density streams. For instance, lower density fuels such as jet fuel typically have a high market cost in relation to other fuels.
Consequently, there is a need for an improved method for reducing the density of hydrocarbon product streams. In addition, a need exists for a more efficient and effective method for blending hydrocarbon product streams to meet density specifications so as to form upgraded blends, wherein some of these upgraded blends are suitable for use as diesels or diesel blend stocks.